If they send 2 emails per subdomain per year, that could easily be 10s of millions which would make the cost per email measured in thousandths of a cent. And I could see the number of subdomains being larger by a factor of 10, maybe more.

Another angle: someone with IT experience needs to manage the system that seems emails, and other engineers need to integrate other systems with the email reminder system. The time spent on engineering could easily add up to thousands per year, if not tens of thousands.

I’m guessing their figure is based on both running costs and engineering costs.

You don’t have to ship a second OS just to containerize your app.

Be ready to replace the disk when the warranty ends.

What’s the point of replacing them? The warranty doesn’t keep them from dying, it just means you get a free replacement. The amount of life left on the drives after the warranty expires depends heavily on how they’re used, and most self hosters are pretty gentle on their drives. I could see replacing the drives that are heavily used, but replacing all drives just because their warranty expired seems like a waste of money and effort.